NARO did testimony, I believe, or publicity last year or the previous before Congress committee opposing Obama’s push to remove tax incentives favorable to royalty owners. They quoted who their average member was and I think how many royalty owners there are. Contact them.
There are numerous current and past class action lawsuits filed on behalf of royalty owners against the various oil/gas producers. While personal information on class members is not necessarily public record in those cases, there is usually some public filing that requires the number of class members to be mentioned. Most names are cross-referenced to a number which may be public. There may be news articles on those cases that mention the potential class member numbers. This is not foolproof and has many pitfalls for accuracy of numbers but gives you an idea of the huge numbers there are all across the USA as most class actions relate to a specific area or situation.
I know of one class action case involving one state (small confined area of state) and one producer that had over 6,000 royalty owners and another class action case involved in the same state but different producer in the same confined area of state that has over 4,000. These royalty owners are not typically other oil/gas producers who may also have a royalty interest as oil/gas producers don’t like to be included in other oil/gas producers lawsuits. Also, many times overriding royalty owners are not members of a class action suit as they also have a stake in the producing wells as an owner of such.